Effects of Grain Source and Concentrate Level on B-Vitamin Production and Absorption in Steers

Hereford-cross steers, each fitted with T-type cannulae in the duodenum and ileum, were used in two trials to determine effects of grain type and concentrate-to-forage ratio on apparent thiamin, niacin, riboflavin and biotin production and absorption. Steers were fed at 2-h intervals and Cr-EDTA and acid insoluble ash were used as markers. Diets were fed as complete pelleted mixtures. Seven steers were used in trial 1 to compare diets containing 15% alfalfa and 85% of either corn, wheat, oat, barley or sorghum. All diets, except the sorghum diet, resulted in a net loss of thiamin in the rumen of steers. Apparent ruminal N and organic matter digestibilities of the sorghum diet were less (P<.05) than those of other diets. Apparent intestinal absorption of thiamin averaged 62.5%. Apparent niacin production in the rumen was substantial (37 to 359% greater than amounts fed) in all steers, as was absorption from the small intestine, which averaged 66.7%. More riboflavin flowed from the rumen than was consumed, indicating ruminal synthesis. Apparent intestinal absorption averaged 22.9% and did not differ significantly among diets. The major site of biotin synthesis appeared to be in the small intestine. In trial 2, the low-concentrate (LC) diet contained 29.5% corn, while the high-concentrate (HC) diet was comprised of 88.9% corn. The LC diet contained more niacin, riboflavin, biotin and crude protein, while the HC diet contained a greater quantity of thiamin. Duodenal concentrations of thiamin, niacin and riboflavin did not differ significantly between diets. The HC diet resulted in increased niacin (P<.01) and riboflavin (P<.05) quantities in feces.